diff --git a/src/core/model/command-line.cc b/src/core/model/command-line.cc
index 56a4c1073..cc77b4072 100644
--- a/src/core/model/command-line.cc
+++ b/src/core/model/command-line.cc
@@ -116,7 +116,7 @@ CommandLine::CommandLine()
     NS_LOG_FUNCTION(this);
 }
 
-CommandLine::CommandLine(const std::string filename)
+CommandLine::CommandLine(const std::string& filename)
     : m_NNonOptions(0),
       m_nonOptionCount(0),
       m_usage()
@@ -181,7 +181,7 @@ CommandLine::Clear()
 }
 
 void
-CommandLine::Usage(const std::string usage)
+CommandLine::Usage(const std::string& usage)
 {
     m_usage = usage;
 }
@@ -198,7 +198,7 @@ CommandLine::Item::~Item()
 }
 
 void
-CommandLine::Parse(std::vector<std::string> args)
+CommandLine::Parse(std::vector<std::string>& args)
 {
     NS_LOG_FUNCTION(this << args.size() << args);
 
@@ -724,7 +724,7 @@ CommandLine::CallbackItem::GetDefault() const
 }
 
 bool
-CommandLine::CallbackItem::Parse(const std::string value)
+CommandLine::CallbackItem::Parse(const std::string& value)
 {
     NS_LOG_FUNCTION(this);
     NS_LOG_DEBUG("CommandLine::CallbackItem::Parse \"" << value << "\"");
@@ -735,7 +735,7 @@ void
 CommandLine::AddValue(const std::string& name,
                       const std::string& help,
                       ns3::Callback<bool, std::string> callback,
-                      std::string defaultValue /* = "" */)
+                      const std::string& defaultValue /* = "" */)
 
 {
     NS_LOG_FUNCTION(this << &name << &help << &callback);
@@ -807,7 +807,7 @@ CommandLine::GetNExtraNonOptions() const
 
 /* static */
 bool
-CommandLine::HandleAttribute(const std::string name, const std::string value)
+CommandLine::HandleAttribute(const std::string& name, const std::string& value)
 {
     bool success = true;
     if (!Config::SetGlobalFailSafe(name, StringValue(value)) &&
@@ -825,7 +825,7 @@ CommandLine::Item::HasDefault() const
 }
 
 bool
-CommandLine::StringItem::Parse(const std::string value)
+CommandLine::StringItem::Parse(const std::string& value)
 {
     m_value = value;
     return true;
@@ -854,7 +854,7 @@ CommandLineHelper::GetDefault<bool>(const bool& val)
 
 template <>
 bool
-CommandLineHelper::UserItemParse<bool>(const std::string value, bool& val)
+CommandLineHelper::UserItemParse<bool>(const std::string& value, bool& val)
 {
     std::string src = value;
     std::transform(src.begin(), src.end(), src.begin(), [](char c) {
@@ -895,7 +895,7 @@ CommandLineHelper::GetDefault<Time>(const Time& val)
 
 template <>
 bool
-CommandLineHelper::UserItemParse<uint8_t>(const std::string value, uint8_t& val)
+CommandLineHelper::UserItemParse<uint8_t>(const std::string& value, uint8_t& val)
 {
     uint8_t oldVal = val;
     long newVal;
diff --git a/src/core/model/command-line.h b/src/core/model/command-line.h
index bbe9b7a56..f387cad8f 100644
--- a/src/core/model/command-line.h
+++ b/src/core/model/command-line.h
@@ -241,7 +241,7 @@ class CommandLine
      *
      * \param [in] filename The source file name.
      */
-    CommandLine(const std::string filename);
+    CommandLine(const std::string& filename);
     /**
      * Copy constructor
      *
@@ -263,7 +263,7 @@ class CommandLine
      *
      * \param [in] usage Program usage message to write with \c \--help.
      */
-    void Usage(const std::string usage);
+    void Usage(const std::string& usage);
 
     /**
      * Add a program argument, assigning to POD
@@ -283,7 +283,7 @@ class CommandLine
      *
      * \param [in] value The argument value.
      */
-    typedef bool (*Callback)(const std::string value);
+    typedef bool (*Callback)(const std::string& value);
 
     /**
      * Add a program argument, using a Callback to parse the value
@@ -300,7 +300,7 @@ class CommandLine
     void AddValue(const std::string& name,
                   const std::string& help,
                   ns3::Callback<bool, std::string> callback,
-                  const std::string defaultValue = "");
+                  const std::string& defaultValue = "");
 
     /**
      * Add a program argument as a shorthand for an Attribute.
@@ -320,7 +320,7 @@ class CommandLine
      *        is parsed, this variable is not modified).
      */
     template <typename T>
-    void AddNonOption(const std::string name, const std::string help, T& value);
+    void AddNonOption(const std::string& name, const std::string& help, T& value);
 
     /**
      * Get extra non-option arguments by index.
@@ -370,7 +370,7 @@ class CommandLine
      *
      * \param [in] args The vector of arguments.
      */
-    void Parse(std::vector<std::string> args);
+    void Parse(std::vector<std::string>& args);
 
     /**
      * Get the program name
@@ -430,7 +430,7 @@ class CommandLine
          * \param [in] value The string representation
          * \return \c true if parsing the value succeeded
          */
-        virtual bool Parse(const std::string value) = 0;
+        virtual bool Parse(const std::string& value) = 0;
         /**
          * \return \c true if this item has a default value.
          */
@@ -450,7 +450,7 @@ class CommandLine
     {
       public:
         // Inherited
-        bool Parse(const std::string value) override;
+        bool Parse(const std::string& value) override;
         bool HasDefault() const override;
         std::string GetDefault() const override;
 
@@ -460,13 +460,13 @@ class CommandLine
 
     /**
      * \ingroup commandline
-     * \brief Extension of Item for strings.
+     * \brief Extension of Item for non-options, stored as strings.
      */
     class StringItem : public Item
     {
       public:
         // Inherited
-        bool Parse(const std::string value) override;
+        bool Parse(const std::string& value) override;
         bool HasDefault() const override;
         std::string GetDefault() const override;
 
@@ -490,7 +490,7 @@ class CommandLine
          * \param [in] value The string representation
          * \return \c true if parsing the value succeeded
          */
-        bool Parse(const std::string value) override;
+        bool Parse(const std::string& value) override;
         ns3::Callback<bool, std::string> m_callback; /**< The Callback */
         std::string m_default; /**< The default value, as a string, if it exists. */
     };                         // class CallbackItem
@@ -526,7 +526,7 @@ class CommandLine
      * \param [in] value The value to assign to \pname{name}.
      * \return \c true if the value was set successfully, false otherwise.
      */
-    static bool HandleAttribute(const std::string name, const std::string value);
+    static bool HandleAttribute(const std::string& name, const std::string& value);
 
     /**
      * Handler for \c \--PrintGlobals:  print all global variables and values
@@ -614,7 +614,7 @@ namespace CommandLineHelper
  * \return \c true if parsing was successful
  */
 template <typename T>
-bool UserItemParse(const std::string value, T& val);
+bool UserItemParse(const std::string& value, T& val);
 /**
  * \brief Specialization of CommandLine::UserItem to \c bool
  *
@@ -623,7 +623,7 @@ bool UserItemParse(const std::string value, T& val);
  * \return \c true if parsing was successful
  */
 template <>
-bool UserItemParse<bool>(const std::string value, bool& val);
+bool UserItemParse<bool>(const std::string& value, bool& val);
 /**
  * \brief Specialization of CommandLine::UserItem to \c uint8_t
  * to distinguish from \c char
@@ -633,7 +633,7 @@ bool UserItemParse<bool>(const std::string value, bool& val);
  * \return \c true if parsing was successful
  */
 template <>
-bool UserItemParse<uint8_t>(const std::string value, uint8_t& val);
+bool UserItemParse<uint8_t>(const std::string& value, uint8_t& val);
 
 /**
  * \ingroup commandlinehelper
@@ -681,7 +681,7 @@ CommandLine::AddValue(const std::string& name, const std::string& help, T& value
 
 template <typename T>
 void
-CommandLine::AddNonOption(const std::string name, const std::string help, T& value)
+CommandLine::AddNonOption(const std::string& name, const std::string& help, T& value)
 {
     UserItem<T>* item = new UserItem<T>();
     item->m_name = name;
@@ -720,14 +720,14 @@ CommandLineHelper::GetDefault(const T& val)
 
 template <typename T>
 bool
-CommandLine::UserItem<T>::Parse(const std::string value)
+CommandLine::UserItem<T>::Parse(const std::string& value)
 {
     return CommandLineHelper::UserItemParse<T>(value, *m_valuePtr);
 }
 
 template <typename T>
 bool
-CommandLineHelper::UserItemParse(const std::string value, T& val)
+CommandLineHelper::UserItemParse(const std::string& value, T& val)
 {
     std::istringstream iss;
     iss.str(value);